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The JWST Discovery of the Triply Imaged Type Ia "Supernova H0pe" and Observations of the Galaxy Cluster PLCK G165.7+67.0 [1]
['Brenda L. Frye', 'Department Of Astronomy Steward Observatory', 'University Of Arizona', 'N. Cherry Avenue', 'Tucson', 'Az', 'Http', 'Massimo Pascale', 'Department Of Astronomy', 'University Of California']
Date: 2024-10
Figure 2. NIRCam color composite image of the central region of G165. G165 is a double cluster with prominent northeastern and southwestern components. Colors follow the prescription in Trilogy (Coe et al. 2012 ) with red showing F444W and F356W, green showing F277W and F200W, and blue showing F150W and F090W. The 21 image systems used in our lens model are labeled. They include the DSFG as Arcs 1a and 1b/1c. The triply imaged SN Ia is labeled as "SN 2a/2b/2c." The orientation and image scale are provided for reference. All known arc substructures are marked, even those not used in the lens model.
After each frame was calibrated, the frames were aligned onto a common astrometric reference frame and drizzled into mosaics with pixel scale 20 mas. The process was similar to that first described by Koekemoer et al. ( 2011 ) but updated to use the JWST pipeline. 28 Mosaics were produced for each filter in each separate epoch. For the six filters in common, all epochs were also combined into a grand mosaic for each filter. All mosaics were aligned onto the same pixel grid based on deep, ground-based CFHT/Megaprime images with good seeing on 2014 May 29 (PI: Nesvadba). The image mosaic was aligned directly onto Gaia DR3 (Gaia Collaboration et al. 2016 , 2023 ) by M. Nonino (2023, private communication). The NIRCam data were aligned onto this grid with residual rms below 2–3 mas and no significant large-scale distortions. Figure 2 shows the central region of G165 in the main NIRCam mosaic.
The NIRCam images were reduced by our team as described by Windhorst et al. ( 2023 ). Briefly, the data were retrieved from the Mikulski Archive for Space Telescopes (MAST), and the latest photometric calibration files were used (pmap_1100). All images were reduced using version 1.11.2 of the STScI JWST Pipeline (Bushouse et al. 2022 ) with an additional correction for 1/f noise by applying the prescription of C. Willott. 27 The ProFound code (Robotham et al. 2018 ) was run, which makes a second round of corrections of other image artifacts in the relevant rows and columns. This step additionally flattens the background and corrects for detector-level offsets, "wisps," and "snowballs" (Robotham et al. 2017 , 2018 ). Since the Windhorst et al. ( 2023 ) publication, improvements in the data reduction techniques have been made by Robotham et al. ( 2023 ) regarding the removal of image wisps by using the wisp-free LW images as priors to identify the outer contours of the real detected objects. Those real objects were subsequently removed from the southwestern images to get a pure wisp image, which was then fully subtracted. This process yields an image noise in the final mosaic that is almost the same in the wisp-removed area as in the surrounding wisp-free areas.
Figure 1. JWST/NIRCam coverage of the G165 field. The background is the r-band negative image from CFHT/Megaprime. Superposed color images show the combined NIRCam data. The pink long-dashed rectangle outlines Epoch 1, and Epochs 2 and 3 are squares that mostly overlap each other but have slightly different rotation angles. The blue square outlines the field of view of previous HST WFC3-IR imaging, which usefully covers a portion of the gap between the two NIRCam modules. The green square frames the field of view adopted to construct the lens model.
The Epoch 1 NIRCam observations were obtained as part of the PEARLS JWST GTO program. The observing date was selected to minimize stray light expected from a nearby bright star. Exposures were taken in four filters in the short wavelength (SW) channel and four in the long wavelength (LW) channel, as shown in Table 1 . Both NIRCam modules collected data. Epochs 2 and 3 of NIRCam imaging were acquired as part of the JWST disruptive DDT program (PID 4446, PI: Frye) to follow the supernova's light curve in each of its three images. In this follow-up program (also summarized in Table 1 ), exposures were taken in six filters using only Module B of NIRCam. The NIRCam observations covered the central region of the cluster including both the northeastern and southwestern cluster components, the three images of the SN, the DSFG, all of the image systems, and other prominent giant arcs. Figure 1 depicts the field coverage overlaid on an extant r-band image using the Canada–France–Hawaii Telescope (CFHT) Megaprime imager.
NIRSpec medium-resolution Micro-Shutter Array (MSA) spectroscopy of the G165 field was obtained on 2023 April 22 as part of the JWST DDT program (PID 4446, PI: Frye). The MSA mask was populated with the positions of the three SN appearances (SN 2a, 2b, and 2c), two of the three images of the SN host galaxy (Arc 2a and 2c), and counterimages of three other image systems (Arcs 5a, 8c, and 9c). The remainder of the mask was filled with other lensed sources, which summed to a total of 42 lensed targets. The observations used the grating/filter combinations G140M/F100LP to cover spectral range 0.97–1.84 μm (rest-frame 0.35–0.66 μm at z = 1.8) and G235M/F170LP to cover 1.66–3.17 μm (rest-frame 0.57–1.1 μm at z = 1.8), both with spectral resolution R ≈ 1000. We also acquired a PRISM/CLEAR spectrum covering 0.7–5.3 μm (rest-frame 0.25–1.9 μm) with R ≈ 20–300 (50–14 Å). All seven of the supplied guide stars were acquired, resulting in especially tight pointing residuals of 1–7 mas and successful pointing even for targets near the edges of an MSA array. The science exposure times were 4420 s, 6696 s, and 919 s for G140M/F100LP, G235M/F170LP, and the PRISM/CLEAR observations, respectively. A three-point nod pattern was selected for each observation, and each MSA slit consisted of three microshutters giving slit height 1 52. MSA slits are 0 20 wide in the dispersion direction, and the long dimension was oriented at position angle 276°.
The Stage 1 calibrated data were retrieved from MAST and reduced using the JWST NIRSpec pipeline, version 1.11.3. 29 Stage 2 and 3 reduction used the JWST pipeline with reference files "jwst_1100.pmap" for all levels of the data reduction with an exception regarding the background subtraction for extended sources, as described below. Saturated pixels and other image artifacts were flagged in the 2D spectra. The NIRSpec IRS2 detector readout mode was used, which largely reduced the 1/f noise. The 2D spectra were wavelength- and flux-calibrated based on the calibration reference data system (CRDS) context. Finally, individually calibrated 2D spectra exposures were coadded, and 1D spectra plus uncertainties were optimally extracted (Horne 1986).
The pipeline background subtraction performed well for single point sources and single small sources, which were fully covered by the aperture. This is because the dithered exposures provided a good "best-fit" background consisting of the intracluster light and/or other underlying extended sources and/or detector offsets. Hence, the resulting NIRSpec flux from the pipeline directly gave the flux for the point/small source.
However, the observations did not include exposures of a separate background field. This made it more of a challenge to estimate the background for sources extending across multiple microshutters. One example is the SN host galaxy Arc 2 and the SN, for which all three microshutters are occupied by sources. For this case, the background template formed in the NIRSpec pipeline comes from the flux through the source shutter in the dithered exposure. This "image from image" background might include some flux from the galaxy, leading to an oversubtraction of the background. A complementary problem is the case for which neighboring microshutters are occupied by different sources. In the "MOS Optimal Spectral Extraction tool from STScI (based on the method from Horne 1986), a source kernel and a polynomial background template are fit at the same time for one source within an MSA slit, based on a spatial window in the 2D spectrum chosen manually by the user, but the software does not support multiple source extraction.
To alleviate some of these issues, a custom-built code was developed to perform the background subtraction. The code is different from the pipeline in that it builds a more locally derived background template. For each pixel, we evaluated the minimum flux of the set of five dithered pixels. Then for each pixel within each spatial column i, the best value for the background was computed by the median value of this minimum flux within a running boxcar 10 spatial columns wide and centered on column i. We found 10 columns to be a good compromise between a smaller median filter starting to encroach on the size of a typical cosmic-ray mask and a larger median filter smoothing out the background features in this wavelength-dependent operation. To cope with image crowding, the code has a multiple source extraction mode that fits multiple source kernels simultaneously for each object along the MSA slit.
Operationally, we ran NIRSpec Stage 2 with the background subtraction task turned off and then applied the custom-built code. The detailed content of this code and its implementation for this data set appear elsewhere (W. Chen et al. 2024, in preparation).
In all, the NIRSpec spectroscopy produced a total of 47 1D spectra. We measured the redshifts from emission- and absorption-line features of each source, as available. The line centers were determined by fitting Gaussians to each spectroscopic line feature using specutils (Earl et al. 2023). Of 47 spectra, 30 produced secure redshifts, which are listed in Table 2. A redshift is considered secure if it has a high-significance detection of two or more spectral features and >2σ level in the continuum. Of these, the highest-redshift galaxy is NS_274, a relatively rare example of a quiescent galaxy for which we measure z sp = 4.1076 ± 0.0023. The highest-redshift multiply imaged galaxy is Arc 5.1a with a redshift measured from Balmer lines from Hα through H detected in emission of z sp = 3.9530 ± 0.0004. The spectroscopic analysis of the NIRSpec spectra of galaxy images at z = 1.78 and z = 2.24 appear in Section 5. (The prefix "NS" stands for NIRSpec, and it precedes all of the NIRSpec-confirmed galaxy images in this study in Table 2.)
Table 2. NIRSpec Spectra ID R.A. Decl. m F200W,obs z sp NS_2 (SNa) 11:27:15.31 +42:28:41.02 ⋯ a ⋯ b NS_3 (SNb) 11:27:15.60 +42:28:33.73 ⋯ a ⋯ b NS_4 (SNc) 11:27:15.94 +42:28:28.90 ⋯ a b NS_6 (Arc 2c) 11:27:15.98 +42:28:28.72 20.26 1.7834 ± 0.0005 c NS_7 (Arc 2a) 11:27:15.34 +42:28:41.05 20.30 1.7833 ± 0.0010 c NS_19 (Arc 5a) 11:27:13.20 +42:28:25.73 25.36 3.9530 ± 0.0004 NS_26 (Arc 8.2c) 11:27:15.89 +42:28:28.90 26.13 1.7839 ± 0.0002 NS_29 (Arc 9c) 11:27:16.11 +42:28:27.99 26.10 1.7816 ± 0.0002 NS_46 11:27:13.87 +42:28:35.67 24.04 2.2401 ± 0.0002 NS_104 11:27:00.84 +42:27:03.65 24.83 3.1109 ± 0.0004 NS_112 11:27:02.46 +42:27:10.22 21.59 0.6227 ± 0.0003 NS_123 11:27:04.64 +42:27:15.24 23.94 1.7874 ± 0.0003 NS_143 11:27:07.64 +42:27:26.22 23.28 1.6322 ± 0.0002 NS_171 11:27:05.82 +42:27:37.10 23.33 1.1787 ± 0.0002 NS_274 11:27:11.67 +42:28:10.70 23.44 4.1076 ± 0.0023 NS_285 11:27:06.82 +42:28:12.88 23.81 0.4466 ± 0.0001 NS_337 11:27:16.28 +42:28:23.59 21.77 1.7810 ± 0.0009 NS_342 11:27:19.46 +42:28:24.79 24.06 1.7664 ± 0.0009 NS_376 11:27:16.91 +42:28:30.62 22.69 0.6840 ± 0.0001 NS_407 11:27:06.62 +42:28:37.80 22.49 1.8553 ± 0.0009 NS_411 11:27:05.17 +42:28:37.75 24.28 1.2456 ± 0.0007 NS_477 11:27:04.23 +42:28:56.36 23.00 1.8524 ± 0.0009 NS_481 11:27:13.66 +42:28:57.12 24.35 1.3236 ± 0.0002 NS_505 11:27:18.95 +42:29:06.64 24.40 1.2888 ± 0.0002 NS_511 11:27:18.69 +42:29:08.42 24.13 3.3255 ± 0.0006 NS_548 11:27:13.21 +42:29:30.75 23.14 0.8161 ± 0.0003 NS_610 11:27:00.97 +42:26:48.25 22.77 0.6628 ± 0.0004 NS_969 (Arc 1a) 11:27:13.92 +42:28:35.43 24.90 2.2355 ± 0.0003 NS_1115 11:27:03.09 +42:29:07.03 23.61 2.0585 ± 0.0007 NS_1477 11:27:16.65 +42:27:23.57 21.04 0.7203 ± 0.0003 Notes. NS numbers refer to the MSA slit identifications assigned when the observations were designed. Positions are object positions as measured on NIRCam images. a The photometry is presented by J.D.R. Pierel et al. (2024, in preparation). b This spectroscopic redshift is presented by W. Chen et al. (2024, in preparation). c Polletta et al. ( The photometry is presented by J.D.R. Pierel et al. (2024, in preparation). This spectroscopic redshift is presented by W. Chen et al. (2024, in preparation). Polletta et al. ( 2023 ) used LBT/LUCI spectra covering observed wavelengths 950–1370 nm to measure z = 1.782 for Arc 2a and z = 1.783 ± 0.002 for the average of Arcs 2a and 2b, fully consistent with the NIRSpec redshifts. Download table as: ASCIITypeset image
The three SN H0pe spectra are of high quality. Most prominent is the requisite detection of the Si ii λ6355 absorption feature blueshifted to ∼6150 Å, closely followed by the detection of the [Ca ii] λ λ 8498,8542,8662 IR triplet "CaT," among other spectroscopic features. The spectrum of the SN, the SN classification as Type Ia, and the measurement of the spectroscopic time delay will appear in a different paper (W. Chen et al. 2024, in preparation). The spectra of the SN host galaxy Arcs 2a and 2b had preexisting redshifts, both based on the joint detection of [O ii] λ3727 and the 4000 Å and Balmer breaks (Polletta et al. 2023). The new NIRSpec spectra give the first redshift for Arc 2c, whose value matches that of Arc 2a (Table 2). Nearly 20 spectroscopic features are detected as well as the 4000 Å and Balmer breaks. Somewhat remarkably, six different lensed sources have the same redshift as the SN. Their images are shown in Figure 3, and their spectra are presented in Figure 4. This redshift is interesting because it also coincides with the strongest peak in the photometric redshift distribution after the cluster redshift, as described in Section 3.2.
Figure 3. NIRCam color image centered on one of the SN host images 2c (Arc 2c), along with three other galaxies spectroscopically confirmed to be at z = 1.78 in this study. Our lens model predicts for Arcs 2, 8, 9, and NS_337 to be situated within 33 physical kiloparsecs in the source plane. The NIRSpec spectra of these arcs appear in Figure 4. Colors follow the prescription in Trilogy (Coe et al. 2012) with red showing F444W and F356W, green showing F277W and F200W, and blue showing F150W and F090W. Download figure: Standard image High-resolution image
Figure 4. NIRSpec spectra of lensed sources at z ≈ 1.78. Wavelengths are in the observed frame, and the ordinate shows F λ in units of 10−19 erg s−1 cm−2 Å−1. The G140M spectrum is plotted in green and the G235M spectrum in blue. Detected lines are marked. The images to the right of each panel show the respective source with MSA slit positions overlaid and are oriented north up, east left. The color rendering is the same as Figure 3. The microshutter depicted in blue is the one from which the spectrum was extracted. The spectra are presented in order of star formation activity with more quiescent sources with weaker Hα emission lines and stronger 4000 Å and Balmer breaks at the top to emission-line sources with multiple nebular emission lines at the bottom. These six sources uncover a diverse set of galaxy properties all contained in this single high-redshift galaxy overdensity. The spectrum for Arc 2a appears brighter and redder than the one for Arc 2c owing to the slit being better centered on the source. Download figure: Standard image High-resolution image
Arc 1b/1c had a previously measured z sp = 2.2357 ± 0.0002 (Harrington et al. 2016). The NIRSpec spectrum (NS_969) gives the first redshift of its counterimage, Arc 1a, and the redshifts agree. A second spectrum (NS_46) in an adjacent MSA microshutter (0 46 northwest) is redshifted by ∼1400 km s−1 relative to Arc 1a. Both spectra, shown in Figure 5, have strong nebular emission lines and starburst attributes. Section 5.2 describes their nebular and stellar properties.
Figure 5. NIRSpec spectra of Arc 1a (=Arc NS_969, top) and a nearby object (Arc NS_46, bottom). Wavelengths are in the observed frame, and the spectra are in F λ in units of 10−19 erg s−1 cm−2 Å−1. The G140M spectrum is plotted in green and the G235M spectrum in blue. Detected lines are marked. The images to the right of each panel show the respective source with MSA slit positions overlaid, and are oriented north up, east left. The color rendering is the same as Figure 3. The objects were observed in the same triplet of MSA slits with Arc 1a in the left segment and Arc NS_46 in the middle segment, neither perfectly centered in the respective segments. The blue outline in each image shows the microshutter from which the spectrum to its left was extracted. Download figure: Standard image High-resolution image
In some cases, most notably for Arcs 2a, 2c, and Arc NS_337, aperture losses are expected because the MSA slit coverage is smaller than the source size. To account for this shortfall, for each filter bandpass, synthetic photometry was computed based on the NIRSpec PRISM spectra, which provided continuous coverage over the wavelength range of all eight NIRCam bands. We then compared our results to the NIRCam photometry integrated over the entire source. We refer the reader to Section 5 for details.
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